The present invention relates to a bleaching detergent composition containing sodium percarbonate incorporated therein.
Detergents for clothing with a bleaching effect have been used by preference in recent years because they can effectively remove the dirt of sebum adhered to clothing as well as the dirt of stains caused by black tea, coffee, fruits, blood, etc.
Bleaching agents may be divided into oxygen-base ones and chlorine-base ones. The former are in wider use because they do not cause fading even when applied to colored clothing or clothing with patterns and have no offensive smell, of which sodium percarbonate is a representative one.
However, sodium percarbonate has several disadvantages. It is poor in storage stability as compared with other oxygen-base bleaching agents such as sodium perborate, etc. and tends to undergo decomposition during storage to reduce its effective oxygen content. This tendency is particularly marked when sodium percarbonate is incorporated into detergents for clothing. Further, with the recent trend toward eliminating phosphorus-containing ingredients from household detergents, zeolite is generally incorporated as a builder to such detergents. This causes the problem of the decomposition of sodium percarbonate being promoted by the catalytic action of zeolite.
Accordingly, it has been eagerly awaited to develop a technique which can improve the storage stability of sodium percarbonate when incorporated into detergents and to develop a bleaching detergent composition excellent in storage stability.
A known method of stabilizing sodium percarbonate is to coat it with paraffin, polyethylene glycol, borates, (Japanese Patent Application Kokoku (Post-Exam. Publn.) No. S61-4879), boric acid, silicates, perborates, pyrophosphates, etc.
Another proposed method is, in preparing sodium percarbonate, to add phosphates, silicates, ethylenediaminetetraacetate, nitrilotriacetate, etc. as a stabilizer in the hydrogen peroxide solution used in the preparation.
However, even the sodium percarbonate obtained by the above-mentioned methods does not show a fully satisfactory storage stability when incorporated into detergent compositions.
A known method of coating sodium percarbonate with boric acid is that disclosed in British Patent No. 1,575,792. However, sodium percarbonate simply coated with boric acid is not fully satisfactory in its stability, and a further improvement of the stability is desired.
The present inventors have made extensive study to improve the storage stability of sodium percarbonate when incorporated into detergents. As a result, it has been found that when sodium percarbonate is coated with boric acid and an alkali metal silicate, an exceedingly highly stabilized sodium percarbonate can be obtained as compared with that coated with boric acid alone.
British Patent No. 1,575,792 describes that the coating agent may contain, besides boric acid, compounds which have an effect of stabilizing peroxides, and particularly mentions alkali metal salts of phosphoric acid and silicic acid as the example of such compounds; but it describes nothing of the specific coated products or the method for coating. Japanese Patent Application Kokai (Laid-open) No. S59-193,999 discloses a bleaching detergent composition incorporated with a sodium percarbonate whose surface has been coated with a coating agent containing at least both a boric acid salt and an alkali metal silicate, and describes the use of boric acid and sodium silicate in combination in its Comparative Example; but it gives no detailed description of the method for practicing it, and the effect of said combined use is not remarkable.
In coating sodium percarbonate with boric acid and an alkali metal silicate, if boric acid and the alkali metal silicate are dissolved together in a solvent such as water, a gel-like precipitate of silicon oxide may be formed though depending on the mixing ratio or the concentration. This phenomenon is particularly marked at high concentrations. In dissolving boric acid and an alkali metal silicate together, therefore, it is not always possible to adopt any composition and any solution concentration of the coating agent as desired.
In producing a stabilizing sodium percarbonate by spraying an aqueous solution of boric acid and an alkali metal silicate onto sodium percarbonate particles to coat the particles therewith, when boric acid and the alkali metal silicate are dissolved together in water, silicon oxide will precipitate in the form of gel if the concentrations of the two are high, as described above (cf. Referential Example 1), and such an aqueous mixture cannot be sprayed to coat sodium percarbonate particles. Thus, when boric acid and an alkali metal silicate are dissolved together in water, the concentrations of the two should be low. However, when the concentrations are low, a large amount of thermal energy is required for drying the sodium percarbonate particles after spraying, which is uneconomical, and moreover a long time is required for the drying, leading to the decomposition loss of active oxygen in sodium percarbonate. To practice the spraying in an industrially advantageous way, therefore, the concentrations of boric acid and of the alkali metal silicate are desirably as high as possible; but sufficiently high concentrations cannot be used since, as described above, gel-like precipitate is formed if boric acid and the alkali metal silicates are dissolved together in water in high concentrations. This is a serious problem.